In Phage, don't sit in a loop once we've read all messages from an agent
Summary: Ref T2794. The logic is intended to read "loop until we're done processing messages", but is currently missing a break;. This doesn't affect behavior yet, just makes us eat way too much CPU.
Test Plan: Saw time phage remote --hosts db001-16 status drop from 99% CPU time to <10%.
Reviewed By: chad
Maniphest Tasks: T2794
Differential Revision: https://secure.phabricator.com/D17387